1. Field of the Invention
The present invention relates to a gear mechanism, and more particularly, relates to a gear mechanism which can rotatably drive, for example, one driven member by two different inputs.
2. Description of the Related Art
A lens drive mechanism provided in a lens barrel of camera, for example a focusing lens drive mechanism for carrying out focusing, may have a structure that allows focusing both by a motor-driven automatic operation and by a manual operation. In Japanese Unexamined Patent Publication (J.U.P.P.) No. H10-153731, a planetary gear mechanism is disclosed which includes a first through third gears to form a focusing lens drive mechanism, in which the first gear of this planetary gear device is connected to a focusing lens, the second gear is connected to a motor, and the third gear is connected to a manual operation ring. This focusing lens drive mechanism rotatably drives the first gear either by rotatable driving of the motor or by rotation of the manual operation ring, thus the focusing lens moves in the optical axis direction, whereby the focusing is carried out. According to the focusing lens drive mechanism of the above-mentioned J.U.P.P. No. H10-153731, during auto focusing through rotatable driving of the motor, the third gear is in the fixed state because of load applied toward the manual operation ring side, and the focusing lens is driven by differential rotation at the planetary gear mechanism. Furthermore, during manual focusing through rotation of the manual operation ring, the second gear is in the fixed state because of the load applied toward the motor side, and the focusing lens is driven by differential rotation at the planetary gear mechanism.
According to the focusing lens drive mechanism of the above-mentioned J.U.P.P. No. H10-153731, the load applied toward the manual operation ring side has been utilized during auto focusing, whereas the load applied toward the motor side has been utilized during manual focusing, in order to obtain a differential rotation output at the planetary gear mechanism. However, because there is no means for either adjusting the load on the manual operation ring side nor the load on the motor side, in the case where the load on the motor side is larger than that on the manual operation ring side, when the motor rotates and the rotational torque is transmitted to the planetary gear mechanism, the third gear connected to the manual operation ring cannot remain in a fixed state due to the smaller load on the manual operation ring side, and the focusing lens may not be able to be driven by the planetary gear mechanism.
Furthermore, since the radial dimensions of the planetary gear mechanism of J.U.P.P. No. H10-153731 is large since the sun gear, the planetary gear and the crown gear thereof are provided on the same plane, a gear mechanism using a planetary bevel gear disclosed in J.U.P.P. No. H07-333699 can be utilized therein. The gear mechanism according to J.U.P.P. No. H07-333699 has an input/output gear and output gears positioned coaxial with the input/output gear, engaged with each other via planetary bevel gears and a pair of sun bevel gears; hence, minimizing the outer circumferential size of the gear mechanism, which further achieves downsizing (miniaturization) of the lens barrel into which such a gear mechanism is incorporated. However, according to the gear mechanism of J.U.P.P. No. H07-333699, since a rotational shaft of each planetary bevel gear is inserted into, and supported by, a small hole formed in the inner peripheral portion of the input/output gear, due to the difference in size between the small hole and the rotational shaft, rotational play (i.e., eccentric rotation) occurs during rotation of the planetary bevel gears. This further causes irregular backlash against the engaging sun bevel gear, making it difficult to obtain a rotational output with high accuracy.